1. Field of the Invention
This invention relates to a control system for an automatic vehicle transmission, more specifically to such a system for improving toughness against (avoidance of the occurrence of) excessive thermal load in the vicinity of the exhaust pipe of a vehicle equipped with an automatic transmission having a manual gearshift mode.
2. Description of the Prior Art
Automatic vehicle transmissions equipped with both automatic and manual gearshift modes have recently been developed. The automatic gearshift mode is implemented by an automatic gear ratio signal output means that determines the gear ratio by retrieval from predetermined gearshift characteristics based on vehicle speed and engine load and outputs a signal representing the determined (retrieved) gear ratio. The manual gearshift mode is implemented by a manual gear ratio signal output means that outputs a signal representing the designated gear ratio selected by a manual operation of the vehicle driver.
In the automatic gearshift mode, the gear (shift position or gear ratio) is retrieved from a predetermined shift pattern (gearshift characteristics) using the vehicle speed and the throttle opening (engine load) as address data. The so-determined gear (ratio) is established by energizing/de-energizing electromagnetic solenoid valves of a hydraulic circuit (gearshift mechanism) to shift up or down automatically.
In the manual gearshift mode, the gear (ratio) is selected (designated) by the driver according to the driver's intention. As a rule, automatic shifting is not conducted. A system has been developed, however, that makes an exception to this rule in order to prevent a vehicle driver operational error from causing vehicle operation in an over-revved state. Specifically, an over-rev prevention shift pattern (forced upshift line) having the vehicle speed and the throttle opening as parameters is defined and a forced upshift is effected when this line is exceeded.
An example of this technology can be found in Japanese Laid-open Patent Application Hei 8(1996)-312774. During high-load operating conditions, this earlier-developed technology moves the forced upshift line toward the low-vehicle-speed side to make upshift easier.
The whole point of the manual gearshift mode is, however, to allow the vehicle driver to select (designate) the gear (ratio) according to his or her intention and desire. Forced upshift, a form of automatic gearshift, detracts from this feature and makes driving less fun for the driver. As such, it lowers the product's appeal to customers.
On the other hand, when a forced upshift line is not established, an operating mistake on the part of the vehicle driver may lead to prolonged operation in an over-revved condition. When this happens, the temperature of the exhaust pipe downstream of the catalytic converter rises to increase the thermal load on nearby components. Conceivable measures for avoiding this problem include installing baffle plates between the exhaust pipe and surrounding components, increasing the clearance between the exhaust pipe and the nearby components, and adopting a double-wall exhaust pipe structure. Since all of these are impractical due to technical requirements and high costs, however, a more realistic solution has been desired.